U.S. patent application number 11/201279 was filed with the patent office on 2006-09-07 for method and system for defect detection.
Invention is credited to Michael J. Darwin, Yonghang Fu, Yongqiang Liu.
Application Number | 20060199287 11/201279 |
Document ID | / |
Family ID | 36944581 |
Filed Date | 2006-09-07 |
United States Patent
Application |
20060199287 |
Kind Code |
A1 |
Fu; Yonghang ; et
al. |
September 7, 2006 |
Method and system for defect detection
Abstract
A method for inspecting objects such as semiconductor wafers. A
staging platform and an optical platform are arranged so that the
object may be staged and its surface scanned by optical equipment
situated on the optical platform. During the scanning process, the
surface is illuminated with light of a plurality of wavelengths,
each strobed at a predetermined rate so that multiple images may be
collected using time and frequency multiplexing. The multiple
images are stored in a database for analysis, which includes
processing selected ones of the multiple images according to one or
more algorithms. The defect-detection algorithms used for each
object are determined by referenced to a predetermined or
calculated defect detection protocol, then a defect mask is created
for each pixel in the images that is suspected to be defective. The
defect mask is then compared to threshold parameters to determine
which if any of the suspected defects should be reported.
Inventors: |
Fu; Yonghang; (Plano,
TX) ; Liu; Yongqiang; (Plano, TX) ; Darwin;
Michael J.; (Beaverton, OR) |
Correspondence
Address: |
SLATER & MATSIL, L.L.P.
17950 PRESTON RD, SUITE 1000
DALLAS
TX
75252-5793
US
|
Family ID: |
36944581 |
Appl. No.: |
11/201279 |
Filed: |
August 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60658914 |
Mar 4, 2005 |
|
|
|
Current U.S.
Class: |
438/16 |
Current CPC
Class: |
G01N 21/9501
20130101 |
Class at
Publication: |
438/016 |
International
Class: |
H01L 21/66 20060101
H01L021/66; G01R 31/26 20060101 G01R031/26 |
Claims
1. A method of inspecting an object, comprising the steps of:
providing an optical inspection system comprising a staging
platform, an optical platform, and a computing facility, the
computing facility comprising a database for storing programs and
collected data; staging the object; scanning the object by
illuminating at least a portion of the surface of the object and
capturing at least one image created by light reflected from the
surface of the object; storing data representing the at least one
image captured in the scanning step; selecting a defect detection
protocol comprising at least one defect detection algorithm;
analyzing the collected data using the selected at least one defect
detection algorithm; building at least one defect mask, wherein
each defect mask corresponds to a specific portion of the object
surface being inspected and has a value determined by a weight
assigned to the at least one defect detection algorithm; and
determining the existence of a defect based on the defect mask.
2. The method of claim 1, wherein the object is a semiconductor
wafer.
3. The method of claim 1, wherein the at least one defect detection
algorithm is a plurality of defect detection algorithms.
4. The method of claim 3, wherein the plurality of defect detection
algorithms are used substantially in parallel.
5. The method of claim 3, wherein at least one of the plurality of
defect detection algorithms is not used until after another of the
plurality of defect detection algorithms has been used.
6. The method of claim 5, wherein the plurality of defect detection
algorithms are run serially.
7. The method of claim 5, wherein the step of determining the
existence of a defect comprises comparing the at least one defect
mask to a predetermined threshold is performed prior to using all
of the plurality of defect detection algorithms.
8. The method of claim 7, further comprising the step of
terminating inspection process in the even that the at least one
defect mask is beyond the predetermined threshold.
9. The method of claim 1, wherein the step of scanning the object
comprises illuminating a surface of the object with light from a
plurality of sources, each source emanating light at wavelength
different than that of the other sources.
10. The method of claim 9, wherein the step of scanning further
comprises separating the reflected light so that multiple images
may be captured.
11. The method of claim 9, wherein the scanning step further
comprises strobing the light from at least one of the plurality of
light sources.
12. A system for inspecting the surface of an object, comprising:
an optical platform for scanning the surface; and a computing
facility for analyzing data collected when the surface is scanned,
wherein the computing facility comprises a plurality of defect
detection scheme modules for analyzing the collected data and a
defect detection scheme manager for selecting which of the
plurality of defect detection scheme modules to apply to the
collected data; and wherein the analysis comprises applying the
selected defect detection scheme modules and building at least one
defect mask, wherein each defect mask corresponds to a specific
portion of the object surface being inspected and has a value
determined by weights assigned to each of the defect detection
modules applied to the collected data.
13. The system of claim 12, wherein the optical platform comprises
a plurality of light sources, each source emanating light at
wavelength different than that of the other sources.
14. The system of claim 13, wherein at least one of the light
sources is strobed at a pre-determined rate.
15. The system of claim 13, wherein the optical platform further
comprises a camera having a plurality of charge-coupled devices
(CCDs) for capturing images and means for separating incoming light
into component wavelengths and directing each component to one of
the plurality of CCDs.
16. The system of claim 13, wherein the collected data is separated
into subsets, each subset corresponding to an image created by
light of a certain wavelength.
17. The method of claim 16, wherein defect detection scheme manager
determines which of the plurality of defect detection modules, if
any, will be applied to each data subset.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/658,914 filed on 4 Mar. 2005, the disclosure of
which is hereby incorporated by reference.
TECHNICAL FIELD
[0002] The present invention is directed generally to the field of
defect detection, and more specifically to a method and system for
inspecting objects such as semiconductor wafers or printed circuit
boards to detect defects using an automated inspection system that
yields a high degree of statistical confidence in the result, thus
ensuring that as many defects as possible are discovered, and also
that non-defective products are not erroneously identified for
discard or repair.
BACKGROUND
[0003] There are many kinds of electrical and electronic devices
that are widely available for scientific, business, and
consumer-oriented applications. Rapid advances in technology have
allowed their use to migrate from universities and large
institutions to small businesses and homes. Computers are now
popular, even for use by children, and a myriad of different
telephones, televisions, games and gadgets may now be found in
almost every household in the country. The new technology has not
only made such applications possible, but has also lowered the cost
of electronic devices to the point where they can be produced in
great numbers and are easily affordable.
[0004] A great many components used in building electronic products
are currently mass-produced despite the fact that their successful
manufacture depends on fabrication to extremely precise tolerances.
Semiconductor wafers, for example, and the printed circuit boards
on which they are mounted, require the formation of a huge number
of very small surface structures. These structures are often formed
automatically using mechanical or chemical means, that is, without
direct human intervention. In the case of, for example,
semiconductor wafers, these structures are formed by alternately
removing select portions of a silicon substrate and applying
additional materials or treating with chemical substances to
produce surface structures having desirable properties. These
structures are often so small that they can barely be seen, if at
all, with the naked eye.
[0005] In one manufacturing operation, a material called
photoresist (or simply "resist") is applied to the surface of a
wafer that is being used to make semiconductor chips. FIG. 1 is an
illustration of an exemplary wafer 100, shown in plan view. The
wafer 100 is divided into a number of dice, for example die 105.
The wafer 100 forms a flat edge (or simply "flat") 110 that may be
used as a reference for locating specific points or dice, such as
center point 115 or die 105. Resist may be deposited at center
point 115 and the wafer 100 spun to evenly distribute the resist
over its surface 101.
[0006] When the resist has been spread over the surface, it is
selectively exposed to light emitted through a mask to create a
pattern. The light causes changes in the resist so that when the
surface is later rinsed, some of the resist will be washed away and
some will remain. This forms a series of structures on surface 101
of wafer 100 (see FIG. 2). The wafer can then be treated, for
example with a solution that etches away portions of the surface
not covered with resist. Or additional materials may be deposited
in similar fashion. This process is repeated until the desired
components have been created on the surface of the wafer. It should
be apparent that the structures mad of resist or of other materials
must be correctly formed onto the surface for the production
process to create properly-functioning components. FIG. 2 is a side
view of a small portion of wafer 100, illustrating the presence of
a number of structures formed on surface 101. Although FIG. 2 is a
cutaway view, it is only for illustration and not intended to
represent any specific section of wafer 100. In addition, the
actual size and location of structures 120, 121, and 122 are
dependent on the specific application and their purpose in the
production process. these structures may be formed of developed
resist, or of materials deposited in the surface 101, or formed as
a result of an etching process.
[0007] Because these surface structures are sometimes created in a
series of reversible steps, identifying defects early may mean that
corrective measures can be taken. And ultimately, finished products
require inspection so that defective ones are not used. In the case
of products such as semiconductor wafers, which frequently are used
to for a number of separate components, portions identified as
defective can be discarded while non-defective portions can be
saved for eventual packaging and use. When production is finished
(to an appropriate stage), the dice are separated and each
individual die (along with a number of leads for providing
electrical connections) is encapsulated in plastic to form a chip
(not shown). Once manufactured, the chip will be programmed to
perform one or more of the many functions for which they are used
in electronic devices.
[0008] As should be apparent, a wafer therefore must undergo a
fairly-large number of manufacturing steps before it is completed.
During manufacturing, it also undergoes a corresponding number of
test and inspections of various types. Although humans can and do
inspect such products during the manufacturing process, often with
the aid of a microscope or similar device, automated inspection
systems are frequently desirable because they can perform the
inspection much faster and, in some cases, more reliably. Optical
inspection systems may be used in this role. Optical inspection
systems, in general, capture images of the object being inspected
after the object's surface has been illuminated by some form of
light energy. The images may be examined by operators, and for this
purpose may undergo some form of enhancement. Captured images,
however, are often converted into digitized form for computer
analysis.
[0009] This analysis may be done in a variety of ways. The images
in digitized form may also be stored for future reference or
converted back into a human-readable visual image. In general,
computer analysis of captured images relies on the relative
characteristics associated with each of a number of picture
elements, or pixels. These pixels may be separately evaluated
because they each represent the light received and converted into
an electrical charge by one of many small photo-sensitive devices
that are housed within a camera. To create a visual image, the data
collected in this way by each of these individual pixels is
aggregated to create a picture. Computer analysis is more flexible,
because it can evaluate the pixel data more precisely and in a
variety of ways. The captured image of a semiconductor wafer being
evaluated may, for example, be compared to a previously-captured
image of a `perfect` wafer (which may have been generated by a
computer rather than captured with a camera). Instead of the
so-called golden-image comparison, some systems employ a die-to-die
or frame-to-frame comparison. In these types of analyses, defective
areas are identified simply because they deviate from other areas
of the wafer that should yield a nearly-identical image.
[0010] Although other inspection methods may be employed, optical
inspection has become very popular in electronics manufacturing
operations and is widely used. Existing systems are far from
perfect, however. For example, all optical imaging systems are
limited in resolution by fundamental optical principles related to
the wavelength of light, numerical aperture of the apparatus used,
and by the overall geometry of the system. As components decrease
in size, inspection tools are continually pushed to identify
defects at or below their optimum optical resolution. In addition,
even in an inspection process that simply compares a newly-captured
image against a theoretically perfect reference, random variations
can lead to noise sources in both the reference and newly acquired
image thus leading to an overall reduction in defect detection
sensitivity.
[0011] In addition, the analytical approach used is typically
applied individually as the specific configuration and setup of
hardware, software, and design strategy permits. Each of the defect
detection schemes in current usage has its own strengths and
weaknesses, and, depending on the defect signature, applicability.
As a consequence, these approaches must often utilize complex
filtering schemes in an attempt to reduce erroneous defects,
sometimes at the expense of overall system resolution.
[0012] Needed then, is a methodology for more efficiently
performing automated defect detection that provides greater
statistical confidence in the result but does not greatly reduce
system resolution. The present invention provides just such a
solution.
SUMMARY OF THE INVENTION
[0013] To overcome the deficiencies in the prior art described
above, the present invention provides an improved design for an
optical inspection system. The present invention provides an
improved design for maximizing sensitivity in defect detection
while statistically increasing robustness. In one aspect, the
present invention is a method for inspecting an object, and
specifically a structure-bearing surface of an object, by scanning
the object with an optical platform having one or more light
sources for illuminating the object's surface and a camera for
capturing one or more images of the illuminated object. The method
also includes determining which of a number of available defect
detection schemes to the data, or to subsets of the data, to
determine within the capability of each scheme the presence of
defects. Each scheme is assigned a weight value, and the method
also includes defining one or more defect masks and building each
mask by including in its value the weight associated with any
defect detection scheme that identifies a defect. Preferably, there
is a defect mask associated with each pixel of an image
corresponding to an area on the surface of the wafer. After
applying the defect detection schemes, the defect mask or masks are
compared to a predetermined threshold and a confidence level in the
existence of a defect in the area associated with the defect mask
is thereby determined.
[0014] In another aspect, the present invention is a system for
performing optical inspection. The system includes an optical
platform having an image collection tool. The image collection tool
is automated such that it can handle patterned semiconductor
wafers. The image collection tool includes a sensor capable of line
or area scanning of sample wafers across a predetermined
electromagnetic wavelength range (in particular in the visible
light spectrum). The sensor can be set up in parallel to perform
multiple independent measurements of the same sample. The analysis
tool is composed of one or more computers with data acquisition
capability set up in parallel. The image collection and analysis
tool (of specified resolution) is setup in a semiconductor process
line after a key process step such as lithography. Images are
collected and analyzed for each channel and one or more defect
masks are created based on the results of the analysis. The defect
mask or masks may then be compared to a predetermined threshold so
that defects may be identified with greater accuracy.
[0015] In yet another aspect, the present invention is a system for
inspecting the surface of an object including an optical platform
for scanning the surface and a computing facility for analyzing
data collected when the surface is scanned. The computing facility
comprises a plurality of defect detection scheme modules for
analyzing the collected data and a defect detection scheme manager
for selecting which of the plurality of defect detection scheme
modules to apply to the collected data. During the analysis the
selected defect detection scheme modules are applied and at least
one defect mask corresponding to a specific portion of the object
surface being inspected is assigned a value determined by weights
assigned to each of the defect detection modules applied to the
collected data. The value of the defect mask is then compared to a
threshold value to identify any surface defects.
[0016] A more complete appreciation of the present invention and
the scope thereof can be obtained from the accompanying drawings
and detailed description of the presently-preferred embodiments of
the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] For a more complete understanding of the present invention,
and the advantages thereof, reference is made to the following
drawings in the detailed description below:
[0018] FIG. 1 is an illustration of an exemplary wafer, shown in
plan view.
[0019] FIG. 2 is a side view of a small portion of the wafer of
FIG. 1, illustrating the presence of a number of structures formed
on the wafer's surface.
[0020] FIG. 3 is an illustration of an optical inspection platform
configured in accordance with an embodiment of the present
invention.
[0021] FIG. 4 is a simplified block diagram illustrating the
relationship of selected components of an optical inspection system
according an embodiment of the present invention.
[0022] FIG. 5 is a flow diagram illustrating a method of inspecting
the surface of an object according to an embodiment of the present
invention.
[0023] FIG. 6 is a simplified process flow diagram illustrating a
method according to one embodiment of the present invention.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0024] The present invention is directed to a method and system for
performing inspections using an optical platform for collecting
images of an object and then processing the images in a manner that
produces a higher degree of confidence that the defects involving
features formed on its surface, if any, have been properly
identified. The present invention exploits multiple independent
detection schemes and correlates them using a defect mask to
increase detection sensitivity while decreasing unwanted nuisance
defects. The method and system of the present invention are
especially advantageous when applied to the inspection of
semiconductor wafers and printed circuit boards during the
production process.
[0025] The present invention will now be described in such an
embodiment, that is, one useful for inspecting semiconductor wafers
during the manufacturing process. The method and system of the
present invention may, for example, be used to inspect wafers to
which photoresist has been applied and developed to ensure that the
photoresist has been properly developed. The present invention may,
of course, also be applied elsewhere in the manufacturing process.
By the same token, the principles involved may be useful in other
types of inspection as well, such as in the manufacture of printed
circuit boards.
[0026] Semiconductor wafers that are being inspected are normally
staged, that is, placed on a platform where they are held
stationary or moved in a way that facilitates the inspections. FIG.
3 is an illustration of an optical inspection platform 300
configured in accordance with an embodiment of the present
invention. Platform 300 includes a base 305 for support, and stage
310 on which the object to be inspected, in this case wafer 100,
will be placed. Other objects may, of course, be inspected in like
fashion. Enclosure 315 may be present to physically shelter the
object being inspected and the optical platform 350, or to allow
control of the inspection environment. Wafer 100 may be placed onto
the platform by a human operator or by mechanical means, such as a
robotic arm (not shown). Guides 320 or other suitable structures
are formed in stage 310 to retain wafer 100 in the proper location
for inspection.
[0027] Optical platform 350 is mounted above stage 310 on travel
assembly 330 in such a way as to allow movement in one or more
directions. Travel assembly 330 includes support rails 332 and 334,
which are movably attached to enclosure 315 and operable to move
upward and downward, raising and lowering optical platform 350.
Travel arms 336 and 337(not visible in this view), are likewise
mounted on rails 332 and 334 such that travel in a front-to-back
direction is facilitated. Finally, optical platform 350 is mounted
on travel arms 336 and 337 in such a way as to facilitate
side-to-side movement. Travel assembly 330 thereby permits optical
platform to be moved in any direction required to complete its scan
of wafer 100. Preferably, a series of coordinated electric motors
(not shown) are used to operate the various components of travel
assembly 330. A computing device 360 may be used to coordinate
motor operation to yield the desired direction of travel.
[0028] Optical platform 350 includes a camera 355 for capturing
images of the surface being inspected. Camera 355 generally
includes a number of charge-coupled devices (CCDs) as well as means
for separating incoming light into different frequencies and
directing each frequency to one or more of the CCDs (not shown).
This, in effect, permits a number of different images to be
captured at the same time. The incoming light in this case, of
course, has been reflected from the surface of the object being
inspected. Optical platform 350 also includes a plurality of light
sources for illuminating the surface. In the configuration of FIG.
3, this includes a coherent light source 360 and two diffuse light
sources 365 and 370, each of which emit light at a different
frequency. Computing device 360 is also used to control the
operation of the light sources 360, 365, and 370, and of the camera
355.
[0029] FIG. 4 is a simplified block diagram illustrating the
relationship of selected components of an optical inspection system
400 according an embodiment of the present invention. Although
portions of some of these features have been described above, there
will not necessarily be a one-to-one correspondence between
components illustrated in FIG. 3 and the functional blocks of FIG.
4. In the embodiment of FIG. 4, inspection system 400 includes an
inspection stage 410 and a computing facility 450. The inspection
stage 410 includes an optical platform 415, which includes any
number of illumination sources for illuminating the wafer during
inspection and one or more image-capturing devices for capturing
images of the illuminated wafer. Inspection system 400 also
includes a travel platform 420, which is the electromechanical
system that produces the relative motion, if any, between the wafer
and the optical platform as is desired during the inspection. As
alluded to above, this will normally involve movement of the
optical equipment relative to a stationary wafer, but this is not
necessarily the case. Travel platform 420 may also be used to
properly position the wafer and the optical equipment even if there
is no relative movement present when image capturing occurs.
[0030] In a preferred embodiment, the optical platform 415 includes
light sources sufficient to emit diffuse light in at least two
different wavelengths and coherent light in a third. These various
light sources are then strobed as the optical platform 415 is moved
in a scanning motion so that the camera captures numerous images of
the wafer surface. Together, these images will form a composite
image that can be evaluated for defects. The use of strobing and
multiple wavelengths allows the system to capture multiple images
of the same area during a single scanning movement.
[0031] The computing facility 450 of inspection system 400 includes
a number of standard components including a central processing unit
(CPU) 455 and a database 460. While each of these components has
been represented as a single entity in FIG. 4, in another
embodiment they may also comprise a number of different physical
entities. By the same token, CPU 455 and database 460 may in some
cases also be used to perform activities other than those
associated with the inspection system.
[0032] Computing facility 450 also includes a number of specific
modules that bear upon the operation of the inspection system 400
according to the present invention. As with the database and CPU,
their function may be performed by one or more dedicated
components, or by components that also perform other functions as
well. The defect detection scheme manager 455 determines which
defect detection scheme or schemes should be used examining a
particular wafer. A set of detection schemes to be used on a
particular wafer may be referred to as a defect detection scheme
protocol. In accordance with the present invention, multiple defect
detection schemes are programmed to be used on the same data set,
running in parallel or serially to identify defects. By the same
token, a particular defect detection scheme may be used to analyze
data sets associated with all or only a portion of the
semiconductor wafer or other object being inspected.
[0033] Computing facility 450 also includes a suite 470 of
individual detection scheme modules, here represented by modules
470a, 470b, and 470n (indicating that any number ay be present. In
a preferring embodiment, defect detection module suite 470 includes
module for two-dimension histogram reference inspection,
two-dimension histogram neighbor frame inspection, neighbor frame
zero crossing inspection, neighbor frame medium inspection,
neighbor frame plane inspection, neighbor frame statistic
inspection, neighbor die zero crossing inspection, neighbor die
medium inspection, neighbor die plane inspection, and neighbor die
statistic inspection. These defect detection scheme modules are
simply preferred; they are not required and others may be present
as well.
[0034] In accordance with this embodiment of the present invention,
a mask definition module 475 for defining the mask associated with
each pixel or other defined image area. When the defect detection
scheme protocol has been completed (or completed up to a
pre-determined point), mask comparison module 480 is used to
compare the currently-defined mask to a predetermined threshold.
This comparison is made to determine whether the pixel is
determined, within a range of confidence defined by the threshold,
to be defective. Communication platform 490 includes transmitting
and receiving equipment for communicating with a network and any
peripheral devices associated with the system 400.
[0035] FIG. 5 is a flow diagram illustrating a method of inspecting
the surface of an object according to an embodiment of the present
invention. As before, the method will be described in terms of
semiconductor inspection although it is applicable in other
environments as well. At START, it is presumed that an appropriate
stage and optical platform has been provided, and configured to
operate according to the present invention. In addition, the
inspection hardware is in communication with a computing device
that has been appropriately programmed and, in this embodiment,
containing the modules described in reference to FIG. 4, above. The
method begins with the selection of a wafer for inspection (step
505).
[0036] Wafer selection may be based any one or more of several
criteria. Not-uncommonly, it is an arbitrary selection based only
on meeting a requirement for selecting a certain number of samples
from with in a given production run. In other applications, all
wafers (or other objects) are inspected. In still other cases the
specific object to be inspected may be chosen based on some
previously-defined or calculated inspection criteria. For example
if a wafer has previously failed an inspection and remedial action
has been taken, the wafer may be automatically selected for
inspection at subsequent opportunities.
[0037] However the selection is made, the process continues with
physically staging the object (step 510). Staging may be
accomplished in-line or off line. For in-line inspection,
appropriate images are captured while the object is still in the
normal production line or location. Note that in reciting the
claims of the present invention, this will still be considered
`staging` even though the system in reality simply waits to begin
until the object has reached an appropriate position. When staging
is off-line, a separate staging platform is typically provided so
that the selected object may be manually or by robotic arm or
similar device may be moved into position for inspection. (See, for
example, FIG. 3.)
[0038] When the wafer or other object has been staged, it is
scanned by the optical equipment (steps 515 and 520). A scan
preferably includes one, but may include multiple passes with the
inspection equipment. Note that in a preferred embodiment, the
optical equipment (examples of which having been described above)
moves with respect to the wager but the wafer may also move.
Naturally, it is the relative movement that is of consequence. In
some applications, no relative movement at all is required, but
this is the exception rather than the rule. The relative motion of
the object and the optical equipment during scanning, if performed,
may be accomplished by lateral or angular movement. That it, the
optical equipment may be mounted on a travel assembly (as in FIG.
3), or it may be fixed so that it des not change location but
simply swivels, sweeps, or rotates, as necessary to perform the
required scan.
[0039] The optical equipment preferably includes a camera and one
or more sources of illumination, as explained in connection with
FIG. 3 above. In a preferred embodiment, the camera and
illumination sources are mounted together on a platform that moves
relative to the staged wafer. The illumination sources include a
coherent light source and a more diffuse light source, the former
for creating a point, line or grid pattern, as desired, on the
surface of the object being inspected and the latter for
illuminating the entire surface or a large portion thereof. For the
camera to capture an image created by each individual light source,
each of the multiple illumination sources use different wavelengths
of light or are strobed on and off at different times, or both.
When different wavelengths are used, the camera is operable to
separate the different images by wavelength.
[0040] Whatever the configuration of the optical platform, the
process continues with the illumination of all or part of the
surface being inspected (step 515). Note that this step may
actually included several illumination steps (not shown
individually) in accordance with the specific design parameters
involved. While the surface is being illuminated, images formed by
reflected light are captured (step 520). The image-capturing step
520 will include the collection of an appropriate number of images
depending on the types of illumination used. The captured image or
images are then digitized (step 525) and stored on an electronic
storage device (step 530).
[0041] Next, the inspection system determines the defect detection
scheme protocol for the wafer being inspected (step 535). The
protocol, as alluded to previously, may be the same for each wafer
being inspected, though there may be reasons to individualize the
protocol as well. The defect detection scheme protocol for a
particular case may indicate that only one defect detection scheme
is used, but the advantages of the present invention are more fully
realized when a number of schemes are employed. The different
defect detection schemes may be applied to the data at the same
time or one after the other in a pre-determined or random order.
They each may use all or only a portion of the data captured for
the wafer. Naturally, they may also refer to data related to the
wafer that was previously captured and stored, if such data exists.
The defect detection scheme protocol may include an indication of
whether this `history` or only current data is to be used.
[0042] Note in this regard that running two or more defect
detection schemes at the same time means only that they are
permitted to be run at the same time; the capacity of the computing
facility may well determine how many modules may actually be
activated simultaneously. the defect detection scheme protocol may
include an indication of whether the defect detection schemes are
applied serially or in parallel.
[0043] One or more defect detection scheme modules are then
activated (step 540), depending on the indications of the selected
protocol. The module or modules use the data associated with the
wafer as is also prescribed in the protocol. The different data
potions available to the data detection module being run are those
images associated with the different light frequencies or strobe
intervals as described above. For convenience, each unique set of
data will be referred to as having been obtained via a separate
channel, whether this channel was created by using a particular
wavelength or a certain time of capture. Data associated with the
same channel may not all be collected simultaneously, however, as
when a laser line is scanned across the wafer surface and a
composite image is then assembled. When at least one defect
detection scheme has been run using the prescribed data, it
generates a result that is then stored for future use (step 545).
The result will include a listing of those defects that have been
found for each pixel (or some other identifiable data unit). If
dictated by the protocol, the same defect detection module may be
applied to other sets of data as well, and the results stored
(steps not shown).
[0044] A defect mask is then defined for each pixel in the captured
image (step 550). (Note that in an alternate embodiment (not shown)
defect masks are created for other image subdivisions instead of at
the pixel level.) Each algorithm and channel is given a weight that
scales the identified defect pixels and provides the appropriate
detection power. These weights are typically assigned in advanced
but could also be altered during the inspection process. For
example, if image data associated with a particular channel is
determined to be less than optimum, the weight of any results
applying that date could be reduced. This may be done automatically
or upon receiving an appropriate response to an operator query.
Weights, of course, may be positive or negative, with a defect
ultimately indicated by a defect mask value respectively above or
below a threshold. In some cases a combination of results may be
assigned a weight. As a simple illustration, modules 470a and 470b
(shown in FIG. 4) may each be assigned a weight of 3, but if both
indicate a defect is present, a total weight of 7 may be added to
the mask. Or conflicting results between two or more specific
modules may result in a defect mask adjustment greater then the sum
of their individual weights. Other combinations are possible, of
course.
[0045] In one embodiment (not specifically illustrated), the defect
masks are created as follows. A two-byte (HI,LO) defect mask is
defined for each pixel within the image. For each pixel, the
algorithm weights are added together and stored in the LO byte, and
the corresponding added channel weights are stored in the HI byte.
Variable thresholds are defined for the HI and LO bytes thereby
determining the confidence level for the defect identified.
Sensitivity and robustness can be varied as follows: a defect
candidate is considered valid if it occurs in any channel/algorithm
(most sensitive) to a defect candidate is considered valid only if
it occurs in every channel and all algorithms (most robust).
Confidence in detected defects is built, for example, by comparing
multiple color channels and using multiple detection modes for each
channel. Note, however, that while the method of the present
invention is frequently applied using a multicolor system, other
types of systems can be used as well. By the same token, defect
masks are not limited to the embodiment described above.
[0046] However developed, the defect mask associated with each
pixel or other defined area suspected to include a defect is then
compared to a predetermined threshold (step 555). Naturally, if no
combination of defect detection scheme and channel has indicated
the presence of a defect, then the area may be presumed with a high
degree of confidence to be defect-free. For other areas, if the
defect mask is within the threshold, the area is also considered to
be defect free. If, on the other hand, the defect is beyond the
threshold, a defect is reported (step 560).
[0047] At this point it is determined whether additional defect
detection scheme modules are to be applied to one or more sets of
data (step 565). If so, the process returns to step 540 and
activates the appropriate module or modules. Steps 545 through 565
are then repeated. If it is determined at step 565 that no further
defect detection scheme modules need to be activated, then the
process continues with the next step (not shown) as dictated by the
defects reported in step 560. This may include discarding the
wafer, marking certain dice for discard, repair, or simply
returning the wafer to (or continuing) the production process.
[0048] Note that the steps of method 500 are organized in a certain
sequences, but other sequences are possible and in accordance with
the present invention. For example, the mask comparison step 555
and the defect reporting step 560 could be performed only after all
required modules have been applied to all of the indicated data.
The advantage performing these steps multiple times is that the
process may be terminated early if the indicated threshold is
reached before each module is activated. This might be expected in
the case of a severe or very obvious defect.
[0049] In an alternate embodiment (not shown), the value of the
threshold may be dynamically adjusted if certain results are
obtained. Naturally, the system operator may adjust the protocols
and thresholds at any time, but at some times it may be
advantageous to do so automatically, based on the result of a
certain inspection or the cumulative result of a number of
inspections. For example, if the application of certain modules (or
a certain number of modules) to selected data sets produce
conflicting results, the threshold may be adjusted automatically or
in response to a query set to the system operator. In another
embodiment (also not shown), the protocol itself is altered or
replaced with a different protocol based on events such as are
described above. In any embodiment, instead of simply identifying
defects associated with masks that or beyond a threshold value, the
system may also report all (or selected) defects and their
associated mask values so that the system operator may finally
determine which defects should be confirmed.
[0050] FIG. 6 is a simplified process flow diagram graphically
illustrating a method 600 according to one embodiment of the
present invention. Block 605 represents a scanned wafer image,
which may include a plurality of wavelengths of light. The image is
them broken down by wavelength (block 610) into its component
images for separate analysis (blocks 615, 620, and 625). In this
embodiment, channel 1 (data associated with one of the component
images) is subject to a frame-by-frame comparison to an ideal
reference (block 630) and defect candidates are identified (block
635).
[0051] In addition, the channel 1 data is analyzed using a nearest
neighbor frame (NNF) comparison (block 640). A zero point crossing
filter is applied to the result (block 645), and candidate defects
are identified (block 650). In the embodiment of FIG. 6, the
channel 1 data is also analyzed using a nearest neighbor die (NND)
comparison (block 655). A zero point crossing filter is applied to
the result (block 660), and candidate defects are identified (block
665). Other defect detection scheme modules may also be applied.
The data from channels 2 and 3 may also be analyzed in the same
fashion, or by using a different set of defect detection schemes.
The defect candidates from each (or selected ones) of these
analyses are then compared (block 670) and defects that can be
identified with a high degree of confidence are reported (block
675).
[0052] In general, the system of the present invention incorporates
a comparison function, which may be applied using any of several
approaches. Defective pixel candidates from each channel or
algorithm are compared with one another to determine if the
candidate defect passes a predetermined robustness test. In
particular, if a defect only occurs for a certain channel, or
certain algorithm, then it may not be a defect. Conversely, if the
defect occurs in many channels and is detected by numerous
algorithms, then the likelihood of the candidate defect being a
real defect has increased. Application of the present invention
thereby increases manufacturing efficiency by provide a higher
degree of confidence in the accuracy of the defect detection
system, and by reducing the need for any individual defect
detection scheme to employ elaborate filtering mechanisms in an
attempt to reduce erroneous results.
[0053] Note that these examples are for purpose of illustration,
however, and not limitation; other variations are possible. Rather,
descriptions above are of examples for implementing the invention,
and the scope of the invention should not necessarily be limited by
this description. Rather, the scope of the present invention is
defined by the following claims.
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